Car exhaust primarily contains harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC). Environmental concerns and government regulations have led efforts to remove these noxious combustion products from vehicle exhaust by conversion to more benign gases such as carbon dioxide (CO2), nitrogen (N2), and water (H2O). In order to accomplish this conversion, the exhaust gases must pass through a treatment system that contains materials that can oxidize CO to CO2, reduce NOx to N2 and H2O, and oxidize hydrocarbons to CO2 and H2O. Recently, lean-burn gasoline and diesel engines have increased in popularity due to their improved fuel economy. These engines, however, have high amounts of oxygen present in the exhaust gas, which leads to inhibition of the catalytic reduction of NOx.
One solution to this problem has been the use of selective catalytic reduction (SCR). SCR injects an ammonia source, such as urea, into the fuel exhaust to reduce NOx emissions to less harmful N2 gas. The utility of SCR is limited, however, by the frequent need to replace the consumable ammonia source.
An alternative solution has been the use of lean NOx traps (LNTs). LNTs adsorb, store, or trap nitrogen oxides during lean-burn engine operation (i.e., when excess oxygen is present) by contacting the NOx gases with a storage material. The LNT system can then switch to a rich-purge phase by reducing oxygen content and increasing the presence of hydrocarbons. During the rich-purge phase, the nitrous oxide is released from the storage material and, promoted by a reducing catalyst, reacts with hydrocarbons to yield nitrogen gas. Although these traps are effective at adsorbing NO2 from the exhaust of lean-burn vehicles, adsorption of other nitrogen oxides (NOx), also present in the fuel exhaust, is inefficient. LNTs may be composed of one or more platinum group metals (PGMs) such as platinum or palladium to catalyze the oxidation of NO for more efficient adsorption, however high loadings of expensive PGMs are required. As such, there is a significant cost associated with the use of these LNTs. Accordingly, there is a need for a more efficient lean NOx traps to minimize the amount of PGMs required by LNTs.